Note
how the most important action, to
compare, is frozen in the abstract noun comparison. Such nouns are called nominalizations, and excessive nominalization
typifies academic and bureaucratic writing.
To tell a simpler and clearer story, one that is less abstract, we would
use the most important action in the sentence as the verb. In transitive
active voice, the sentence would become

(b) We compared the effects of
pH vs temperature.

And
in transitive passive voice, the sentence would be

(c) The effects of pH and temperature
were compared.

You
might select (a) if you were trying to impress the reader (to a reader who
did not know better, a comparison
was made might sound more important than we
compared) or if you wanted to increase the length of your paper but
lacked additional information or ideas (students learn and love the academic
style because it helps them inflate three pages of ideas into a five-page
essay). Regarding the latter point, note that sentence (a) uses 11 words
while sentences (b) and (c) use only 8. Also note that (b) and (c) but especially
(b) tell a simple story that is easy to understand and that anyone who would
write (a) would probably also use the other characteristics of the academic
style. Also note that while (b) and (c) use the same number of words, the
actor is described in (b) but not in (c).

To
write clearly, scientists should avoid needless nominalizations. In other
words, scientists should not be needlessly abstract and should usually express
the important action in the verb.

But
some nominalizations are necessary or useful. Consider the following sentence:

Fragmentation, grazing, forestry,
and nutrient deposition are decreasing the biological diversity of many
of the earth's remaining semi-natural ecosystems.

Of
the four subjects, fragmentation and nutrient deposition are
the most obvious examples of nominalizations. But converting these abstract
ideas into verbs while maintaining this sentence would be very difficult.
Moreover, the main action in the sentence is not really fragmentation
and deposition but decreasing. Finally, most readers of this
paper would be familiar with the terms fragmentation, grazing, forestry,
and nutrient deposition and would understand that the writer is using these
abstractions as characters in a story. Some characters are reducing biodiversity.
Which characters? Fragmentation, grazing, forestry, and nutrient deposition.
In this example, the abstractions (the nominalizations) enable the writer
to tell a story that would be much too long and complex if he or she expanded
the nominalizations into verbs and clauses.

So,
use nominalizations sparingly. Do not use a nominalization until you have
determined that expressing the nominalization as a verb will not improve
the sentence.